Three-dimensional motion capture discriminates extremely mild cerebellar ataxia from parkinsonism in the upper limbs

Research overview

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Abstract Background Upper-limb motor symptoms in spinocerebellar degeneration (SCD) and Parkinson’s disease (PD) often overlap, making differentiation challenging in mild cases based on clinical observation of the finger-to-nose test (FNT) or the diadochokinesis test (DDKT). This study aimed to identify quantitative kinematic markers that distinguish mild SCD from mild PD using three-dimensional (3D) motion capture. Methods Twenty-six patients with mild SCD, 26 with mild PD, and 26 normal controls (NCs) underwent motion analysis during the FNT-forward, FNT-backward, and DDKT. Key parameters, including segmental velocity ratios during the FNT and rotational angle parameters during the DDKT, were analyzed. Significant parameters were validated in an additional cohort of 20 patients with moderate-to-severe SCD, and 23 with moderate-to-severe PD, and associations with motor rating scores were examined. Results During the FNT-forward, the velocity ratio in the final one-third of the movement was significantly higher in patients with mild SCD than in those with mild PD and NCs and correlated with dysmetria scores in the moderate-to-severe SCD group. During the DDKT, patients with mild PD showed smaller rotation angles and a greater decline over time than patients with mild SCD and NCs. This reduction correlated with akinesia scores in the moderate-to-severe PD group. Conclusion Three-dimensional motion capture identified disease-specific kinematic features not detectable by visual inspection. Mild SCD was characterized by increased terminal velocity during FNT-forward, reflecting subtle dysmetria, whereas mild PD showed reduced rotation amplitude during the DDKT, reflecting early akinesia. These markers may support early differential diagnosis and upper-limb motor assessment.

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